This is a study to assess the efficacy and safety of 3,4-diaminopyridine (DAP) in patients with congenital myasthenic syndromes. DAP is an orphan drug that has been used in other countries for almost 20 years to treat patients with Lambert-Eaton myasthenic syndrome and other diseases with abnormal neuromuscular transmission, but which has not been approved for clinical use in this country. There are several forms of congenital myasthenia that result from abnormal ACh release or deficiencies of the ACh receptor. Treatment of these conditions is limited to the use of cholinesterase inhibitors, which produce temporary improvement in strength in many, but not all patients. The current study was designed to determine if DAP is effective and safe in patients with various forms of congenital myasthenia. During an initial dose-ranging trial, an escalating dosage schedule is used to identify in each patient the lowest, maximally-effective dose that produces no side-effects. Patients are hospitalized on the Clinical Research Unit during the dose-ranging trial to assure close monitoring of responses and to detect side-effects. Adults receive 5 mg DAP qid for two days, 10 mg qid for two days, 15 mg qid for two days and 20 mg qid for two days. For children, the respective doses are 12.5 mg/m2, 25 mg/m2, 37.5 mg/m2 and 50 mg/m2.